Báo cáo khoa học: Recruitment of coregulator complexes to the b-globin gene locus by TFII-I and upstream stimulatory factor doc

Recruitment of coregulator complexes to the b-globin gene locus by TFII-I and upstream stimulatory factor Valerie J. Crusselle-Davis, Zhuo Zhou, Archana Anantharaman, Babak Moghimi, Tihomir Dodev, Suming Huang and Jo ¨ rg Bungert Department of Biochemistry and Molecular Biology, Center for Mammalian Genetics, Shands Cancer Center, Powell Gene Therapy Center, Genetics Institute, University of Florida, College of Medicine, Gainesville, FL, USA Gene expression is regulated at multiple steps involv- ing the relocation of genes in the nucleus, the modifi- cation of chromatin structure and, ultimately, the recruitment of transcription complexes [1–3]. Each step is regulated by transcription factors that interact in a sequence-specific manner with DNA control elements located in promoters or enhancers [4]. These sequence- specific binding proteins recruit coregulators that modify histones, mobilize nucleosomes or recruit components of the basal transcription machinery [5]. Tissue-specific genes are often regulated by tissue- specific activators and repressors that act in concert with ubiquitously expressed transcription factors. The sequential stage-specific expression of the five b-like globin genes is regulated by gene proximal regulatory elements that recruit transcription factors either activating or repressing gene expression [6]. The regulation of globin gene transcription involves the recruitment of chromatin modifying activities that regulate accessibility to subregions of the globin gene locus in a developmental stage-specific manner. High-level expression of the globin genes requires a locus control region (LCR) located far upstream of the embryonic e-globin gene and composed of five DNaseI hypersensitive (HS) sites that are 200–400 bp in size and separated form each other by 2–4 kbp [7–9]. The LCR HS sites function together in a synergistic or additive manner to stimulate globin gene expression [10–12]. There is increasing evidence that transcription of at least some, perhaps highly expressed, genes takes Keywords coregulator; globin genes; locus control region; transcription Correspondence J. Bungert, Department of Biochemistry and Molecular Biology, College of Medicine, University of Florida, 1600 SW Archer Road, PO Box 100245, Gainesville, FL 32610, USA Fax: +1 352 392 2853 Tel: +1 352 273 8098 E-mail: jbungert@ufl.edu (Received 16 May 2007, revised 3 October 2007, accepted 5 October 2007) doi:10.1111/j.1742-4658.2007.06128.x Upstream stimulatory factor and TFII-I are ubiquitously expressed helix- loop-helix transcription factors that interact with E-box sequences and or initiator elements. We previously demonstrated that upstream stimulatory factor is an activator of b-globin gene expression whereas TFII-I is a repressor. In the present study, we demonstrate that upstream stimulatory factor interacts with the coactivator p300 and that this interaction is restricted to erythroid cells expressing the adult b-globin gene. Further- more, we demonstrate that Suz12, a component of the polycomb repressor complex 2, is recruited to the b-globin gene. Reducing expression of Suz12 significantly activates b-globin gene expression in an erythroid cell line with an embryonic phenotype. Suz12 also interacts with the adult b-globin gene during early stages of erythroid differentiation of mouse embryonic stem cells. Our data suggest that TFII-I contributes to the recruitment of the polycomb repressor complex 2 complex to the b-globin gene. Together, these data demonstrate that the antagonistic activities of upstream stimula- tory factor and TFII-I on b-globin gene expression are mediated at least in part by protein complexes that render the promoter associated chromatin accessible or inaccessible for the transcription complex. Abbreviations ChIP, chromatin immunoprecipitation; EPO, erythropoietin; ES, embryonic stem; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; HDAC, histone deacetylase; HS, hypersensitive; LCR, locus control region; MEL, murine erythroleukemia; PRC, polycomb repressor complex; siRNA, short interfering RNA; USF, upstream stimulatory factor. FEBS Journal 274 (2007) 6065–6073 ª 2007 The Authors Journal compilation ª 2007 FEBS 6065 place in so-called transcription factories, specific domains in the nucleus enriched for RNA polymer- ase II and splicing factors [13,14]. A recent study by Ragoczy et al. [15] demonstrates that the LCR medi- ates the association of globin genes with transcription factories during the differentiation of erythroid cells. Observations that the LCR recruits transcription com- plexes before they become detectable at globin gene promoters support the hypothesis that the LCR is the primary attachment site for the recruitment of macro- molecular complexes involved in chromatin structure alterations and transcription [16,17]. Regulation of the globin genes involves the action of many transcription factors, some of which have been characterized in detail. GATA-1, EKLF and NF-E2 are hematopoietic transcription factors that have all been shown to participate in LCR function and b-glo- bin gene expression [6]. In addition to these tissue restricted transcription factors, ubiquitously expressed transcription regulatory proteins such as Sp1, upstream stimulatory factor (USF) and TFII-I have also been demonstrated to regulate globin gene expression [18,19]. Previous studies have shown that the helix- loop-helix protein USF activates b-globin gene expres- sion and interacts with E-box elements located in LCR element HS2 and in the b-globin downstream pro- moter region [20]. TFII-I, another helix-loop-helix pro- tein, interacts with the b-globin initiator sequence and represses b-globin gene expression. TFII-I exerts part of its function by recruiting histone deacetylase (HDAC) to the b-globin gene promoter and rendering the chromatin inaccessible for transcription complexes [20]. The polycomb repressor complex (PRC) has origi- nally been identified in Drosophila, in which it plays an important role in regulating the expression of segment polarity genes, including the Hox gene cluster [21]. Homologous proteins have also been identified in mammalian cells. There are two main PRC complexes, PRC1 and PRC2. PRC2 contains the histone methyl- transferase Ezh2, which methylates lysine 27 on the histone H3 N-terminal tail [21]. This modification is absent or reduced in promoters of transcribed genes. PRC1 interacts with PRC2 and contains subunits that recruit DNA methyltransferases. Current models pro- pose that the PRC2 complex initially represses gene activity by H3K27 methylation. Subsequent interac- tions with the PRC1 complex appear to stabilize the repressed chromatin structure by recruitment of DNA methyl-transferases [22]. In the present study, we demonstrate that USF interacts with coactivator p300 in mouse erythroleuke- mia cells that express the b-globin gene. No inter- actions are detectable between USF and p300 in K562 cells, a human erythroleukemia cell line that does not express significant levels of the b-globin gene. CBP ⁄ p300 interacts with LCR element HS2 and the b-globin promoter in murine erythroleukemia (MEL) cells, but only with LCR HS2 in K562 cells. Further- more, we demonstrate that the polycomb group pro- tein Suz12 associates with the b-globin gene promoter in K562 cells, but not in MEL cells. Inactivation of Suz12 led to a three- to five-fold increase in b-globin gene expression in K562 cells. Our data suggest that TFII-I recruits HDAC3 and the PRC2 complex to the b-globin gene promoter to establish an inaccessible chromatin configuration. Results CBP ⁄ p300 interacts with USF and the b-globin promoter in MEL but not in K562 cells We have shown previously that USF is required for high-level b-globin gene expression in MEL cells [20]. USF functions as a classical transcription factor that is able to stimulate transcription in in vitro transcription systems [23]. Recent data from the Felsenfeld labora- tory have shown that USF is also a critical part of a chromatin boundary in the chicken b-globin gene locus [24]. It was shown that USF interacts with CBP ⁄ p300, suggesting that it can function at least in part by recruiting chromatin modifying activities. MEL cells expressing a dominant negative mutant of USF exhib- ited a reduction in Pol II loading to LCR element HS2 and to the b-globin gene promoter [20]. At the same time, we observed a reduction in the recruitment of CBP and p300 to these sites. To examine in more detail whether USF recruits co- activators to the b-globin gene locus, we first examined the recruitment of different coactivators to regions in the globin gene locus (Fig. 1A). The data show that CBP and p300 are efficiently crosslinked at the tran- scribed b-globin gene promoter and at LCR element HS2 in MEL cells (Fig. 1B). No interactions were detected at the repressed embryonic ec-globin gene promoter (Fig. 1B). In K562 cells, CBP and p300 interact efficiently with HS2 but not with the repressed b-globin gene promoter (Fig. 1C). There is some inter- action between CBP and the expressed e- and c-globin genes in K562 cells but the interaction appears to be less efficient compared to LCR HS2. We next analyzed interactions between USF and p300 in MEL and K562 cells by co-immunoprecipitation (Fig. 1D). The results demonstrate that USF interacts with p300 in MEL cells but not in K562 cells. This Coregulators of USF and TFII-I in erythroid cells V. J. Crusselle-Davis et al. 6066 FEBS Journal 274 (2007) 6065–6073 ª 2007 The Authors Journal compilation ª 2007 FEBS interaction is specific because no interactions between HDAC3 and p300 are observed. Taken together, the data demonstrate that USF interacts with the coactiva- tor p300 in erythroid cells and suggest that it recruits p300 to specific regions in the b-globin gene locus. Interaction of Suz12 with the b-globin gene promoter in K562 but not in MEL cells TFII-I interacts with the b-globin initiator and represses b-globin gene expression in embryonic erythroid cells [20]. We have shown previously that TFII-I interacts with HDAC3 in K562 cells. Polycomb group proteins were originally identified as repressors of gene expression during development in Drosophila. Recently, it was shown that the PRC2 is located at and represses developmentally regulated genes in undifferentiated, embryonic stem (ES) cells [22]. To examine whether PRCs are located at the b-globin gene promoter in embryonic cells, we carried out chro- matin immunoprecipitation (ChIP) experiments using antibodies against Suz12, a component of PRC2, in K562 and MEL cells (Fig. 2). The data demonstrate that Suz12 can be crosslinked to the repressed b-globin gene promoter in K562 cells (Fig. 2A) but not to the transcribed b maj -globin gene promoter in MEL cells (Fig. 2B). We did not detect any interactions of Suz12 with the embryonic e-globin gene promoter. The Suz12 antibody used in these experiments specifically recog- nizes both mouse and human proteins and Suz12 is expressed in both K562 and MEL cells as determined in western blotting experiments (data not shown). Interaction of TFII-I with HDAC3 and Suz12 We previously demonstrated that TFII-I interacts with HDAC3 and recruits this protein to the b-globin gene promoter in K562 cells. In the present study, we ana- lyzed the interaction between TFII-I and HDAC3 in both MEL and K562 cells and show that this inter- action is restricted to K562 cells. We next wished to examine whether the PRC2 complex, which interacts with the b-globin gene in K562 cells, could be recruited to the gene by TFII-I. Co-immunoprecipitation experi- ments demonstrate that TFII-I interacts with HDAC3 in K562 cells, consistent with our previous data, but AB CD Fig. 1. CBP ⁄ p300 interact with the b-globin gene locus and with USF1 in MEL cells but not in K562 cells. (A) Schematic of the organization of the human and mouse b-globin gene loci. The human b -globin locus depicted on top consists of five genes which are expressed in a developmental stage-specific manner in erythroid cells as outlined. The expression of the genes is regulated by a LCR composed of five DNaseI HS and located approximately 15–27 kbp upstream of the embryonic e-globin gene. The murine b-globin gene locus, which is depicted on the bottom, consists of four genes which are expressed either in erythroid cells of the embryonic yolk sac (EY or bH1) or in definitive erythroid cells derived from fetal liver or bone marrow hematopoiesis (b maj and b min ). The murine LCR also contains multiple HS required for high-level globin gene expression. K562 (B) and MEL cells (C) were subjected to ChIP analysis with antibodies against p300, CBP or with a nonspecific antibody (IgG). Purified DNA was analyzed by real-time quantitative PCR with primers specific for LCR HS2 or the e-, c-, b-, b-major or ec-globin gene promoters. (D) K562 or MEL cell extracts were precleared with anti-(rabbit IgG) beads and precipitated with a-USF1, a-p300, a-CBP or a-HDAC3 (as negative control), and complexes were captured by incubation with anti-(rabbit IgG) beads. Complexes were eluted off the beads with Laemmli buffer and incubation at 95 °C for 10 min and loaded onto a 5% Ready gel (Bio-Rad). The membrane was probed with a-p300. V. J. Crusselle-Davis et al. Coregulators of USF and TFII-I in erythroid cells FEBS Journal 274 (2007) 6065–6073 ª 2007 The Authors Journal compilation ª 2007 FEBS 6067 not in MEL cells, in which the b-globin gene is tran- scribed (Fig. 3A). We also detected interactions between TFII-I and Suz12 in K562 cells (Fig. 3B). The interaction between TFII-I and Suz12 is not as efficient as that involving HDAC3 and it is not restricted to K562 cells, because interactions are also detectable in MEL cells (data not shown). Reduction of Suz12 expression in K562 cells increases b-globin gene expression We previously used SMART-pool short interfering RNA (siRNA) reagent from Dharmacon and effi- ciently reduced expression of TFII-I and HDAC3 [20]. Reductions in both TFII-I and HDAC3 expression by more than 80% led to an approximately three-fold increase in b-globin gene expression. To examine whether Suz12 and the PRC2 complex participate in the repression of the adult b-globin gene in K562 cells, we reduced expression of Suz12 by RNA interference. The western blot results demonstrate that siRNA transfected cells reveal a drastic reduction in Suz12 protein levels compared to mock transfected cells or cells transfected with nonspecific siRNA (Fig. 4A). Expression of the adult b-globin gene was increased by three- to five-fold in cells transfected with Suz12 siRNA compared to mock or negative control siRNA transfected cells (Fig. 4B). These results demonstrate that the PRC2 complex, or components thereof, partic- ipate in the repression of the adult b-globin gene in embryonic erythroid cells. We also detected an increase in the expression of the embryonic e-globin gene. However, this increase was not as pronounced as the one seen for b-globin gene expression. The PRC2 complex contains the histone H3K27 specific histone methyltransferase Ezh2 [21]. Ezh2 catalyzes the di- and tri-methylation of H3K27 [25]. Using ChIP, we did not detect high levels of tri- methylated H3K27 at the globin gene locus in K562 cells, consistent with studies from the Blobel labora- AB Fig. 2. Suz12 interacts with the adult b-globin gene in K562 but not in MEL cells. Antibodies against Suz12 and nonspecific IgG were used in ChIP assays using K562 (A) and MEL (B) cells. Quantitative PCR was performed with primers that amplified the promoters of the genes as indicated. A B Fig. 3. Interaction of Suz12 and HDAC3 with TFII-I. (A) Interaction of HDAC3 with TFII-I in K562 but not MEL cells. K562 or MEL cell extract was precleared with anti-(a)-(rabbit IgG) beads and precipi- tated with a-HDAC3 or IgG, and complexes were captured by incu- bation with anti-(rabbit IgG) beads. Complexes were eluted off the beads with Laemmli buffer and incubation at 95 °C for 10 min and loaded onto a 10% Ready gel (Bio-Rad). The membrane was probed with a-TFII-I and then stripped and probed with a-HDAC3 as a positive control. (B) Interaction of Suz12 with TFII-I in K562 cells. K562 extracts were precleared with anti-(rabbit IgG) beads and precipitated with 2.5 lgofa-Suz12 or 2.5 lgofa-IgG (as nega- tive control) antibodies. The complexes were captured by incuba- tion with anti-(rabbit IgG) beads. Complexes were eluted off the beads with Laemmli buffer and incubation at 95 °C for 10 min and loaded onto a 10% Ready gel (Bio-Rad). The membrane was probed with a-TFII-I. The lane labeled control represents a regular western blot for TFII-I with protein extract from K562 cells. Coregulators of USF and TFII-I in erythroid cells V. J. Crusselle-Davis et al. 6068 FEBS Journal 274 (2007) 6065–6073 ª 2007 The Authors Journal compilation ª 2007 FEBS tory [26]. In addition, the association of trimethylated H3K27 was not significantly altered in cells that express, or do not express, the adult b-globin gene (data not shown). Interaction of Suz12 with the b maj -globin gene promoter decreases during activation of the b maj -globin gene in differentiating murine ES cells We next examined the association of Suz12 with the b maj -globin gene promoter during differentiation of mouse ES cells. We previously demonstrated that the adult b-globin gene is expressed at low levels in ES cell cultures incubated for 5 days with erythropoietin (EPO), which mediates the differentiation and prolif- eration of erythroid cells [17]. High-level expression of the adult b-globin gene was observed at day 12 in the ES cell differentiation system. Figure 5A,B dem- onstrates that b maj -globin gene expression is up-regu- lated by more than 30-fold between days 5 and 12. We observed that the association of Suz12, TFII-I and trimethylated H3K27 (H3K27me3) with the b maj -globin gene promoter is high at day 5 but undetectable at day 12 (Fig. 5C). Quantitation of the Suz12 levels at the b maj -globin gene promoter dem- onstrate that the changes between days 5 and 12 are significant. The control experiment demonstrates that interaction of LCR HS2 associated dimethylated H3K4 (H3K4me2) does not change during the course of differentiation. Neither Suz12, nor TFII-I were found to associate with a control region located between LCR elements HS2 and HS3 (data not shown). Discussion We provide evidence that USF and TFII-I regulate b-globin gene expression through the recruitment of coactivator complexes that render the b-globin pro- moter accessible or inaccessible to the transcription complex. USF recruits the histone acetyltransferase p300 to the b-globin promoter and this activity increases the accessibility for transcription factors. TFII-I recruits HDAC3 and the PRC2 complex, which render the chromatin structure inaccessible to the tran- scription complex. Previous data from the Felsenfeld laboratory have shown that USF interacts with the coregulators p300, CBP, SET7 ⁄ 9 and PCAF and perhaps recruits these activities to a chromosomal boundary element [24]. In the present study, we show that p300 and CBP are located at the promoter of the active adult b-globin gene and also at LCR element HS2. USF1 is observed to interact with p300 exclusively in erythroid cells with an adult phenotype. These data suggest that USF1 recruits p300 to the promoter of the active b-globin gene to aid in transcriptional activation. CBP ⁄ p300 is located at LCR HS2 in K562 cells, in which we did not detect interactions between USF and p300. This suggests that the recruitment of CBP ⁄ p300 to the LCR, at least in K562 cells, is mediated by pro- teins other than USF, and potential candidates are GATA-1 and NF-E2, which have been shown to inter- act with CBP [27,28]. Both of these proteins were shown to be required for histone acetylation of spe- cific regions in the globin gene locus. However, the recruitment of CBP ⁄ p300 to the adult b-globin gene AB Fig. 4. Suz12 represses b-globin gene expression in K562 cells. K562 cells were nucleofected with Suz12 siRNA, nontargeting siRNA (neg), or mock transfected. (A) Protein was collected after 2 days and electrophoresed on gels for western blotting. Blots were probed with a-Suz12 in the upper panel and then stripped and reprobed with a-GAPDH for a loading control. (B) Relative b-, c- and e-globin expression in Suz12 knockdown cells. RNA was collected, reverse transcribed, and analyzed by quantitative PCR. Expression is set relative to either non- targeting siRNA (neg) samples or mock transfected cells, with GAPDH as the internal reference. V. J. Crusselle-Davis et al. Coregulators of USF and TFII-I in erythroid cells FEBS Journal 274 (2007) 6065–6073 ª 2007 The Authors Journal compilation ª 2007 FEBS 6069 promoter is mediated by USF. We previously demon- strated that the expression of a dominant negative mutant of USF in MEL cells resulted in decreased interactions between CBP ⁄ p300 with both b-globin gene promoter and LCR HS2, suggesting that, in adult erythroid cells, USF is required for the recruitment of CBP ⁄ p300 to both the b-globin gene and the LCR. Expression of the dominant negative mutant also reduced interactions of RNA polymerase II with LCR HS2 and the b-globin gene promoter [20]. E-box motifs are present downstream of the transcription start sites of both the b-globin promoter and HS2, sug- gesting that USF is important for transcription in both of these regions in adult erythroid cells [29]. Components of PRC2 are expressed at high levels in embryonic tissues and are essential for the earliest stages of vertebrate development. They also have been found to occupy a special set of developmental genes in ES cells that must be repressed to maintain pluripotency and that are poised for activation during ES cell differentiation [30,31]. As the adult b-globin gene is repressed during the early stages of develop- ment and is poised for activation, we reasoned that PCR2 could be involved in repressing b-globin expression at the embryonic and fetal stage of devel- opment. Reducing the activity of Suz12 in K562 cells led to an increase in b-globin gene expression and also a modest increase in e-globin gene expression (Fig. 4). It should be noted that the K562 cells were not induced by hemin, which has been shown to increase the expression of e- and c-globin gene expression in these cells. However, the effect of hemin on globin gene expression in these cells is relatively low [32]. Neverthe- less, it is possible that the PRC complex represses globin gene expression in a differentiation and devel- opmental stage-specific manner. This is consistent with previous findings demonstrating that the PRC2 complex localizes to developmentally regulated genes in undifferentiated ES cells [30], and with our own data demonstrating that Suz12 and TFII-I interact with the b maj -globin promoter during early but not late stages of EPO-induced ES cell differentiation [20] (Fig. 5). AC B D Fig. 5. The interaction of Suz12 with the b-globin gene promoter decreases with increased b-globin gene expression during erythroid differ- entiation of mouse ES cells. (A) RT-PCR analysis of e- and b-globin gene expression during erythroid differentiation of mouse ES cells. RNA was isolated from ES cells incubated in the presence of EPO for 5 or 12 days, as indicated. The RNA was reverse transcribed and subjected to RT-PCR using primers specific for the control genes Rex1 and b-actin as well as the embryonic e- and adult b maj -globin genes. (B) Quanti- tative RT-PCR analysis of b maj -globin gene expression at days 5 and 12 of erythroid differentiation of mouse ES cells. RNA was isolated from the cells at the indicated time points after addition of EPO and subjected to quantitative RT-PCR analysis using primers specific for the b maj - globin gene. (C) Analysis of modified histones, Suz12 and TFII-I interactions with the b maj -globin gene promoter at days 5 and 12 of erythroid differentiation. Cells were collected at the indicated time points and subjected to ChIP analysis using antibodies specific for histone H3 dime- thylated at K4 (H3K4me2), histone H3 trimethylated at K27 (H3K27me3), Suz12, TFII-I and the control IgG. (D) Quantitative analysis of Suz12 interactions with the b maj -globin gene promoter at days 5 and 12 of erythroid differentiation. Cells were taken at the indicated time points and subjected to ChIP with the indicated antibodies. The precipitated DNA was subjected to quantitative PCR using primers specific for the b maj -globin gene. In the left panel, chromatin was precipitated with antibodies specific for histone H3 dimethylated at K4 (H3K4me2) and analysed by quantitative PCR using primers specific for LCR element HS2. Coregulators of USF and TFII-I in erythroid cells V. J. Crusselle-Davis et al. 6070 FEBS Journal 274 (2007) 6065–6073 ª 2007 The Authors Journal compilation ª 2007 FEBS Suz12 has previously been shown to be required for the di- and tri-methylation of H3K27, which are marks for inactive chromatin [33]. We did not observe high levels of trimethylated H3K27 associated with the b-globin gene promoter in K562 cells. However, trime- thylated H3K27 was present at the b maj -globin gene promoter in early stage EPO-induced ES cell cultures (day 5) when the b maj -globin gene is expressed at low levels. This modification was absent in day 12 cultures that express high levels of the b maj -globin gene (Fig. 5). The interactions between TFII-I and Suz12 in K562 cells are weak. This interaction may be unstable or transient in nature. In this context, it is interesting to note that another initiator binding protein, YY1, has also been shown to directly interact with both HDAC3 and the PRC complex [34,35]. Furthermore, YY1 has been implicated in the silencing of e-globin gene expression in adult erythroid cells [36]. Suz12 could also be recruited to the b-globin gene promoter through the interaction with other DNA binding proteins involved in repressing the adult glo- bin gene. One possible candidate is BP1, a homeo- domain-containing protein, which binds to a negative regulatory element in the upstream b-globin gene region and reduces expression [37,38]. It is not known how this protein represses b-globin gene expression. Another corepressor recruited by TFII-I to the glo- bin gene locus is HDAC3. TFII-I and HDAC3 interact with each other exclusively in K562 cells and not in MEL cells. It has been observed that TFII-I and HDAC3 have very similar expression patterns in the developing mouse embryo [39]. Therefore, the results presented in the present study, along with the previous studies, suggest that the transcription activity of TFII- I may be controlled by HDAC3 during early develop- ment. Our data demonstrate that TFII-I functions as a repressor of b-globin gene expression by recruiting HDAC activity to the promoter. This is consistent with observations of the acetylation of the human b-globin promoter region being reduced in erythroid cells with an embryonic phenotype [16,40]. EKLF is present and active in both primitive and adult erythroid cells but somehow is unable to activate b-globin gene expression in embryonic erythroid cells [41]. Perhaps EKLF functions in conjunction with other factors to activate b-globin gene expression and these factors are not present or active in embryonic cells. Alternatively, EKLF requires an accessible chro- matin structure to recruit nucleosome mobilizing activ- ities to the promoter. Initial accessibility may be provided by proteins that recruit histone acetyl or methyl transferases to the adult b -globin gene pro- moter, thereby counteracting the repressive activity of TFII-I. Experimental procedures Cell culture Human erythroleukemia (K562) cells were grown in RPMI 1640 medium supplemented with 10% fetal bovine serum and 5% penicillin-streptomycin. Murine erythroleu- kemia (MEL) cells were grown in RPMI containing 10% fetal bovine serum and 5% antibiotic-antimycotic. Cells were grown at 37 °C and maintained at a cell density of approximately 1–5 · 10 5 cellsÆmL )1 . Mouse ES cells were cultured and induced to differentiate with EPO as described by Levings et al. [17]. ChIP, co-immunoprecipitation Chromatin immunoprecipitation was carried out as described by Leach et al. [19]. Briefly, cells were washed in NaCl ⁄ P i , crosslinked using 1% formaldehyde, and quenched with glycine. After isolation of nuclei and lysis, the crosslinked chromatin was fragmented by sonication to average size fragments of 500 bp. Chromatin fragments were precipitated with specific or nonspecific control (IgG) antibodies. Antibody ⁄ chromatin fragments were subjected to several rounds of stringent washing. The DNA was puri- fied from the chromatin fragments and analyzed by real- time quantitative PCR using pairs of primers specific for the murine or human b-globin gene locus as described by Crusselle-Davis et al. [20] in addition to mouse ec-glo- bin promoter (upstream: 5¢-ATGACCTGGCTCCACC CAT-3¢, downstream: 5¢-TCTTTGAGCCATTGGTCAGC- 3¢); human HS2 (upstream: 5¢-CGCCTTCTGGTTCTGTG TAA-3¢, downstream: 5¢-GAGAACATCTGGGCACAC AC-3¢); human c-globin promoter (upstream: 5¢-CCTTCA GCAGTTCCACACAC-3¢, downstream: 5¢-CTCCTCTGT GAAATGACCCA-3¢); human HS3 ⁄ 2 (upstream: 5¢-GTG ACCTCAGTGCCTCAGAA-3¢, downstream: 5¢-ACCTAT CACAGCACCACACC-3¢); and human glyceralde- hyde 3-phosphate dehydrogenase (GAPDH) promoter (upstream: 5¢-ACGTAGCTCAGGCCTCAAGACCTTG-3¢, downstream: 5¢-GACTGTCGAACAGGAGGAGCAGA GA-3¢). Immunoprecipitation was carried out essentially as described by Crusselle-Davis et al. [20]. The antibodies used in the experiments comprised: monomethyl-histone H3 Lys27 07-448, trimethyl-H3 Lys27, Suz12 07-379 (Upstate, Charlotteville, VA, USA); USF1 (H-86) sc-8983, p300 (N-15) sc-584, CBP (A-22) sc-369, GAPDH (FL-335) sc-25778 (Santa Cruz Biotechnologies, Santa Cruz, CA, USA); Suz12 ab12073, TFII-I ab10464 (Abcam, Cambridge, MA, V. J. Crusselle-Davis et al. Coregulators of USF and TFII-I in erythroid cells FEBS Journal 274 (2007) 6065–6073 ª 2007 The Authors Journal compilation ª 2007 FEBS 6071 USA); and IgG C6409 anti-(chicken IgG); (Sigma, St Louis, MO, USA). RNA interference siGENOME SMART-pool reagents for Suz12 (NM 015355) and siCONTROL nontargeting pool (D- 001810-10-05) were obtained from Dharmacon (Lafayette, CO, USA) and used for transfection as described by Crus- selle-Davis et al. [20]. A total of 10 6 K562 cells were nucleo- fected with siRNA or mock transfected using an optimized protocol for K562 cells (Amaxa Biosystems, Cologne, Germany). Protein and RNA were collected from transfected K562 cells after 48, 72 and 96 h, as described previously [20]. Relative expression was determined using real-time RT-PCR and primers as described by Crusselle- Davis et al. [20]. In addition, human c-globin primers (upstream: 5¢-TGAATGTCCAAGATGCTGGA-3¢, down- stream: 5¢-CATGATGGCAGAGGCAGAG-3¢) were used. Protein isolation and western blotting Protein isolation and western blotting was performed as described by Leach et al. [19]. A total of 20 lg of protein was loaded onto 7.5% or 5% Ready gels (Bio-Rad, Hercu- les, CA, USA), electrophoresed, and transferred to nylon membranes. The detection of proteins on the membranes was performed using the ECL Plus system as described in the manufacturer’s instructions (Amersham Pharmacia Bio- tech, Piscataway, NJ, USA). The primary antibodies used were the same as those used in the ChIP and immunopre- cipitation assays in addition to HDAC3 (H-99) sc-11417 (Santa Cruz). 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Recruitment of coregulator complexes to the b-globin gene locus by TFII-I and upstream stimulatory factor Valerie J. Crusselle-Davis,. whether USF recruits co- activators to the b-globin gene locus, we first examined the recruitment of different coactivators to regions in the globin gene locus